MISSING TRANSVERSE ENERGY MEASUREMENT IN ATLAS DETECTOR:FIRST LHC DATA RESULTS AND IMPORTANCE FOR PHYSICS STUDY

The Large Hadron Collider (LHC) at CERN started its operation at the end of November 2009, first at a centre-of-mass energy of 900 GeV, then, since March 2010, at 7 TeV. During this period the ATLAS experiment has collected a large number of proton-proton collision events, resulting up to now in an integrated luminosity of about 45 pb−1 . miss A very good measurement of the missing transverse energy, ET , is es- sential for many physics studies in ATLAS both for Standard Model chan- nels, as W, Z bosons decaying to τ leptons or top quark decays, and for miss are expected to be the key discovering channels. Events with large ET signature for new physics such as supersymmetry and extra dimensions. A miss measurement in terms of linearity and resolution is crucial for good ET the efficient and accurate reconstruction of the Higgs boson mass when the Higgs boson decays to a pair of τ -leptons. miss in ATLAS with real This thesis describes the first measurement of ET data. miss reconstruction has been The performance of the algorithm for ET widely tested in minimum bias and di-jet events that are not expected to miss miss is calculated from the calorimeter have a genuine ET . In ATLAS ET cell energies, from reconstructed muons energy and from an estimation of the energy lost in dead materials. Since it is reconstructed from everything miss is a complex quantity: particles escaping because seen in the detector, ET of the limited coverage of the detector, presence of dead regions and different miss . A lot of effort has been dedicated sources of noise can produce fake ET miss distribution and to apply selection to understand well the tails in the ET miss signals. cuts to remove fake ET As the ATLAS calorimeters are not compensating, that is the response to hadronic and electromagnetic particles is different, the calibration of the miss perfor- calorimeter cell energies is crucial. With 900 GeV data the ET mance has been studied and it has been found as expected from simulation miss calibration has been validated. at the EM scale.With 7 TeV data the ET miss performance using different calibrations algorithms has been tested, ET confirming the good agreement between data and simulation found at the EM scale. The effect of pileup has also been studied in details. miss commissioning the absolute scale determination To complete the ET is needed. This will be possible only when enough statistics of events with miss will be available. genuine ET First, with the already available statistics, using the standard ATLAS miss scale can be checked from the shape of the W W → lν selection, the ET miss and transverse mass distribution that is calculated from the lepton and ET has only a few percent of background contamination. This method allows to miss scale with a precision of 3% considering only statistical errors set the ET and using less than 50 pb−1 of data at 7 TeV. Then Z → τ τ events will be used where a Z boson decays into a couple of τ leptons, then a τ decays into a lepton and neutrinos and the other one into hadrons and a neutrino. It is possible to reconstruct the invariant mass of the two taus using the reconstructed lepton, the hadronic part of miss in the so-called collinear approximation. From the the τ (τ -jet) and ET miss scale. A invariant mass peak position it is possible to determine the ET detailed study has been done on simulated data to collect a very pure signal miss sample with low backgorund contamination. With this method the ET scale can be fixed with a precision of 6% with an integrated luminosity of about 100 pb−1 at 10 TeV. The first chapter of this thesis is dedicated to the physics at LHC: the theoretical motivation is briefly described. The importance of a very good miss measurement is shown for the study of many physics channels, with ET particular attention to the Standard Model Z → τ τ channel, the search for supersymmetry and the discovery channel A/H → τ τ in the Minimal Supersymmetric Standard Model. The second chapter describes LHC and the ATLAS experiment focusing on the detector commissioning done with the first data at LHC. miss measurement with In the third chapter I document the results of ET miss calculation in ATLAS is given first ATLAS data. The description of ET detailing the different algorithms available, from the most basic to the most miss calibrations are also described. The perfor- refined one. The different ET miss in data is compared to the simulation with careful under- mance of ET standing of the events in tails. The last chapter describes the preliminary results obtained with ATLAS miss scale from W → lν events and also a detailed data to determine the ET miss scale from Z → τ τ events that Monte Carlo based study to set the ET will be used when at least 100 pb−1 of data will be available.